mon: Paxos needs to store the latest version permanently on-disk.

mon: Paxos needs to store the latest version permanently on-disk.

Previously it was only storing this m->latest_value in the stash,
which of course got overwritten. And then when somebody tried to read
it back, it failed!
Instead, require that the message include the regular version (not
just the stashed version), which the previous commit provides. And then
write the regular version to disk alongside the stash.

This set of procedures still suffers from some of the same disk consistency
issues as we recently fixed in slurping, but it's better than it was, and
fixing those would require a good deal more work.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>